Prior techniques for generating long electron beams, as for wide or long webs, surfaces or work pieces demanding uniform and simultaneous electron beam irradiation, have included the longitudinal single filament type gun constructions disclosed, for example, in U.S. Pat. Nos. 3,702,412 and 4,100,450 of common assignee herewith. The core technology of the above patents involves a single filament which is positioned over the entire length of the electron gun, and which supplies all of the electrons over the full length of the machine. Multiple single filament guns, mounted side by side (in parallel) over the full length of the machine can also be used for increased beam power. The filament is generally surrounded by plates at positive voltage ("Extractor Voltage", Vex) to attract electrons, with one plate, the control grid, having an opening slit for some of the electrons to be accelerated in the desired direction. The open slit is positioned parallel to the filament and extends the full length of the gun.
Experience and theory show that such a long single filament gun has good linear beam uniformity characteristics for an electron beam current dynamic range of about 20:1. A 500 mA electron gun accelerator using a single filament gun, for example, has good linear beam uniformity over a range of 25 mA to 500 mA. At a dynamic range greater than 20:1, however, the linear beam uniformity at low current begins to deteriorate and becomes worse as the dynamic range increases. For example, for the same 500 mA gun accelerator, the beam uniformity at 10 mA is better than the uniformity at 5 mA, and so on. This phenomenon is due to the effect of the filament voltage drop on extraction voltage. The longer the filament length, the greater the voltage drop. The filament is heated by an AC power supply which produces an alternating (AC) voltage across the filament V.sub.fil, such voltage being added to or subtracted from the DC grid extractor voltage V, depending on the time in the AC cycle, as later explained; and for long filaments, results in current variations along the filament that produce non-uniform electron extraction.